Method for electrolytic deposition of chromium



'20 A further advantage-oi. mys inventionds'that Patented May 19, 1936 METHOD FOR ELECTROLYTIC DEPOSITION OF CHROMIUM Floricel A. Rojas, New York, N. Y., assignor to Rojas Chemical Works, Inc., New York, N. Y., .a corporation, of New York No Drawing. Application May 15, 1931,

' Serial No. 531,745.

3 Claims. (c1- 204-1) This invention relates to theelectrolytic deposition of chromium, aswell as the methods and means employed in connection therewith;

The object of this invention is to provide meth- 5 ods and means for chromium plating which ofler' decided advantages in efllciency, flexibility and economy over the methods heretofore suggested or used. V r

A further object of the inventionis to improve the art of chromium electroplating by making use of the advantages which are inherent to the principles of secondary deposition. Among these; advantages are fine grained deposit; throwing power; maximum distribution of thedeposit; maximum range of curr'entdensity-and current. quantity in bright plating; "the elimination of critical values in the physical andjchemical characteristics ofthe bath, such as temperature-and I 1- th dically-bo a chemically active. conditiontwjIn this state it is. a powerful chemical reducingagent chemical balance. v.

it permits the electroplating of chromium directly on the metals of which the objects to be plated" are madewithout-preliminarycoating with co per andnickelas is customary at the present time. i 1

Aiurther advantage ot my invention-lathe:

ability of rendering heavy depfosits of 'chromiumi either by the use'oi high currentdensityf'without limitation of time, when hard deposits are desired; or low current'density-also without limita'e tion of time when soft and ductile*deposits are desired.

A further object of the invention is I to such an electrolyte for use in connection with the "electrolytic deposition Of Qhl'Ol-fllllfll" as'will be capable of rendering'anyoi the'iore going'results commerciallyxpractical'.

Further objects" of the'invention' moreiully hereinafter: v v

The invention consists in th' "method of-electros 4 0 plating, as well as the electrolyteemployed in: connection: therewith, all- 1 as will" be more fullyhereinafter set forth and flnally 'pointed out inthe appended claims.

In the chromium electroplating processes heretofore'proposed orutilized; the principleswofprimary deposition have been employed :notwith I standing the decided advantages that would'be' obtained by utilizing secondary deposition m'thv ods. i

"m :deposition refers particularly-trio electroplating ot metals fromnthe first ionization of the electrolytayitis'limitedito; the.- plating. out-or two, substances; one} of them=beingf always hydrogenand the otherr-the metalawhich;

is intended to be deposited.

Secondary deposition, however, refers to;the electroplating of metals by a subsequent ioniza-- 1 'tion of the anodic ions created by the first ionization of the electrolyte. It involves the platingiout of more than two substances, plating out '(be- 5 sides hydrogen and the metal in .question) .a

third metal whichI will hereinafter; refer to asa 1 Katho1yst.= i For the sake of conciseness, the word Katholyst willbe used throughout thisdiscour se to signify, 10 I such element or substance which, when forming;

part of an electrolyte, will, by the-passage oithe current, separate at the cathode, but'will'; neither remain'nor there maintainits chemicaljand phys:

ical .constitution: without reacting rapidly-with 15,

electrolyte." 3 ,1. AKatholyst may beiurther defined ,as being such a'substance. which is readily reduced 1cawhich reacts rapidly with'the depolarizer present,

becoming thus reoxidized into a chemicallylpas-,-;

Katholyst separated 'at the cathodeimmediately 6 rea ts with h ec d itql a h d ox e, and thus reestablishes the negative charge :ot;

the cathode. The negat've ion, of the ionization 1 r .,on w ount. of i s ;meliallic--con1ponent,

is in itseli an unstable compound. As soon as the negative charge of the ,cathode ;is}reestabe,-

' lished by oxidation or the vlKatlicilyst, 'the vn ietallic component of the; anion lbecomescpositive mi grates to .thecatho'deiianfl.. i fih r i lg t tive electricity, neutralizes the negative charge. 4o

of that electrode, and platesiitseli{there as; a

electro deposition'of silver fror'n the double yanid' so of silver'and potassium; tlie' electro' depodtio'n "01 f copper from the double cyanide i'-=-'"copper and "sodium andnthe electrordepositiono ni-ekel -iroin the double sulphaterbt-nickel andsammoniuma f1he chemical characteristics of chromium, on 55 v account of the double nature of this metal, differ basically from those of silver, copper and all the noble and semi-noble metals. Chromiumpaltakes of both of the properties of the metals and the non-metal elements in that it is capable of forming either a base or an acid radical. With chromium, salts are formed such as sodium chromate, or chromium sulphate. In the first instance, chromium supplies an acid and, in the second, it supplies a base. Such chemical propensity is not the behavior of true metals in the strict sense of the term.

. Due to this double nature, chromium is capable of forming either positive or negative ions. The e of the current through a solution of chromium sulphate for instance, will discharge the positive electricity of the cathions and precipitate free atoms at the negative electrode. In a solution of sodium chromate, however, chromium will form negative ions, and consequently no direct depos't of metallic chromium will result from this electrolyte.

The use of such complex electrolytes such as the solutions either of the double chromate of sodium and chromium or thedouble chromate of potassium and chromium containing free chromic acid, will, while complying with the principles of secondary deposition, fail to-give the desired results. In the presence of such strong bass as are those of the alkaline'metals, the chromium kmswillbedestinedtocarrythenegativechargm of electricity.

This behavior of chromium in the presence of the alkaline bases probably accoimts for the fact that for the electro deposition of chromium the principls of secondary deposition have not, to my knowledge, been heretofore utilised.

Itis among-the special Purposes of my present invention to overcome the disadvantagesand limitations imposed by primary deposition methods withrespecttochromimnplatingand toobtain the advantages inherent to the principle of secondary deposition for electroplating of chromium. This I amenabled to do afterlextensive experiments extending over a considerable periodoftimebyfindingamaterialtobeusedasa Katholyst heretofore-unused as such in connection with chromium plating or any other metal d'epmition.

I have discovered that when aluminum in any chemical form, but more particularly gelatinous hydroxide, is used-in the construction of an electrolyte intended for chromium plating it atfords a marked improvement over the use of any addition agent and will deposit chromium even in the presence of the very strong bases of the alkaline metals in the electrolyte.

One noteworthy advantage that this new electrolyte has over those using sulphuric acid as an addition agent, is the inexhaustibility of the I aluminum in its Katholystic action.

Aluminum, in its Katholytic cycle as above stated, constantly changes its chemical constitution but does not change its quantity. In sulphete electrolytes, however, the sulphuric acid forms the anions and is constantly being exhausted by-its chemical combination with the anode. It is evident, therefore, that the newelectrolyte using aluminum for'its operation will not require the professional skill and constant attention that the sulphate bath does for the maintenance of its working conditions.

It is a requisite, for the best results, in the construction of the Katholystic chromium bath to. p

tion of trivalent chromium.

'tothecomplexelimiical found that by supplying have no other inorganic acid present in the electrolyte other than chromic acid.

, Were there to be introduced sulphuric acid besides the aluminum Katholyst some complications would arise in theionization of a number of dis- 5 tinct salts of both the chromic and sulphuric acids and aluminum oxides and chromous oxides as the bases, resulting in erratic performance of the bath. It is, therefore, desirable to keep the Katholystic chromium bath free from mother inorganic acid.

Organic acids or other. suitable organic substances intheof the strong powerofchromicacidwillbechangedinto carbonic, acid and water and theconsequent forma- As above stated, the electrolytes for chromimn v plating in general employ acid radicals as addition agents- These addition agents promote the electrolysis ofwaterhaving thchyd'rogenionsfw wastefully diverting a great percentage of the currentatthesacrifheofcurrentin the deposition of the chromium metal. Further-1 more,thiseifectis proliiicofnegativeconditions inchromiumplating andexplainstoagreatextent the instability of mrrow rurge of current density, low cln'rent-efliciency, critical working points, and many otherofthe malts which rembr chromium plating by mums of the now ganployed an arduous and It is well known that'in ins fine grained deposits of metals, maximum covering power and great muse current are rmumin electrolytes ln'vins complex ions the metal ion concentration is very low. Low metal ionconcentrationsuchaswouldoccurinhighly dilute solutions however is not sufllcient mimictoinsmeanemcientdeposit' --v,-.. ofthe current will soon remove the metal in which are present and there is insuiiicient metal available to be discharged. An inferior, burned and spongydepositistheresult. 'Iherefore,means45 mustbeprovidedtoobtainsuchanamountof' dissolved metal salts as will fm'nish asuiilcient .suppiyofionsnearlyasfastasth eyareremoved.

of time, I have found that the electrolyte be.

comes more and more efllcient and the quality 00 of electroplating improved. I have, therefore, salts or solution of the following constituents- .moscnoucrom as an electrolyte may be obtained which initially, as well as throughout its life, will' give the desired results. With the electrolyte thus produced the actual metal cation concentration is very small and the amount of metal salt dissolved in the solution is very large. Thus, fine grained deposits are obtained because, under-the influence of a high-current, the rapid growth of each, crystal of metal is prevented from the scarcity of metal cations and a greater number of small crystals is the result. However, by changing the current ,density, 1. e. lowering it, in the electrolyte and ing maximum throwing power in complex cation solutions is explained by the phenomenon of polarization. While I advance such explanation I desire to have it understood that I do not desire to be limited or confined thereto, as such explanation is advanced as a theory for what benefit it may be. Complex electrolytes, on account of their low cation concentration, offer a marked resistance to the passage of the current to those points of the cathode which are nearer to the anode. At those points polarization is greater because of their comparatively greater proximity to the anode. These nearer points are exposed to higher current densities than the far points of an irregular cathode surface and the discharge of the cations at the near point is faster than the supply from the dissolved metal salts in their vicinity. This propensity has a tendency to equalize the distribution of the secondary current and consequently produces a more uniform thickness of distribution of the deposit upon the entire contour of the irregular cathode surface.

It is evident from the foregoing that a greater range of current density is possible in plating from complex or low cation concentration than from simple or high cation concentration solutions. In the complex cation of electrolytes designed for secondary deposition the unretained cation or Katholysts, according to accepted theory, will plate out ahead of the metal which it is. intended to obtain, and consequently the cathode efliciency decreases as the current density increases. This automatic check on the deposit of the desired metal on the high points permits the use of a greater range of current densities in secondary than in primary depositions. Hence, in accordance with my invention, the range of current densities is extremely wide in chromium plating, whereas in processes heretofore employed it is confined to a narrow range of current density. able to control at will the physical characteristics insofar" as finish, grain, etc., of the chromium deposit.

Obviously, as a result, I am While I have confined my discussion hereinhefore to complex ions in electroplating incident to secondary deposition, it will .be readily apparent that the same principles will apply to tertiary deposition, etc. I have in fact obtained chromium plates having a special texture by tertiary deposition from the three chromates of sodium, aluminum and chromium, and alsoby the tertiary deposition of the tetra chromates of potassium, sodium, aluminum and chromium. Therefore my invention is, broadly stated, directed to the utilization of complex ion electroplating, as distinguished from the primary deposition methods heretofore long found and thought alone to be of commercial utility.

Having now set forth the objects and nature of my invention and having described the details thereof and the formulae incident thereto, what I claim as new and useful and desire to secure by Letters Patent is:

1. A chromium plating bath containing chromic acid but devoid of other inorganic acids and consisting essentially of a water solution of the oxides of chromium and aluminum.

2. A bath for the electrodeposition of metallic chromium consisting essentially of a water solution of chromic acid free from other inorganic 3. The method of electrodepositing metallic chromium which comprises electrolyzing a bath consisting of a water solution of chromic acid free from other inorganic acids and having aluminum hydroxide dissolved therein.

FLORICEL A. nous. 

